Linking molecular motors to membrane cargo

Abstract

Three types of motors, myosins, kinesins, and cytoplasmic dynein, cooperate to transport intracellular membrane organelles. Transport of each cargo is determined by recruitment of specific sets of motors and their regulation. Targeting of motors to membranes often depends on the formation of large multiprotein assemblies and can be influenced by membrane lipid composition. Motor activity can be regulated by cargo-induced conformational changes such as unfolding or dimerization. The architecture and function of motor: cargo complexes can also be controlled by phosphorylation, calcium signaling, and proteolysis. The complexity of transport systems is further increased by mechanical and functional cross-talk between different types of motors on the same cargo and by participation of the same motor in the movement of different organelles.

Figure 1. “Multitasking” motors

The scheme illustrates the multiplicity of cargos transported by the two type V myosins (Myo2 and Myo4) in budding yeast and kinesin-1 in mammals. Where known, components of membrane attachment protein complexes and their modes of membrane interaction are indicated. Membrane attachment often depends on lipid anchors, such as geranylgeranyl groups in the case of Rabs (Ypt11, Ypt31/32 and Rab3, Rab6 and Rab27a), and myristoyl and palmitoyl groups for the yeast vacuole protein Vac8 [19]. Transmembrane proteins, such as Inp2 [1], the small GTPase Miro [34], APP [2], and various receptors such as AMPAR [20], GABA_A [51] and TrkB [23 ″] can also serve as a part of the motor attachment complex, often in conjunction with adaptors. Animal nuclei can be linked to microtubule motors through proteins of the Syne/nesprin family, large cytoskeletal linkers that pierce the outer nuclear membrane [52 ″], or through nuclear pore complexes [35]. Compartment-specific motor receptors can attach to unique regions on the surface of the motor’s CBD, as has been described for Myo2 [76]. Kinesin-1 also uses different binding sites for different cargo: it is a heterotetramer of two heavy and two light chains, and some adaptors such as Milton, HAP1, and GRIP1 interact with the heavy chains [20,34,51], while others, such as JIP1 and CMRP-2 [23 ″,40], bind to the light chains. Possible competition for the same binding site on the motor has also been described [1], underscoring the need for regulation and coordination in multitasking. For some organelles, such as the ER, motor receptors are still elusive. In mammalian cells, the transmembrane ER protein kinectin was proposed to act as a kinesin receptor, but its importance was later disputed [33].

Figure 2. Cargo-dependent regulation of motors

The schemes illustrate the two main mechanisms identified to date where cargo can regulate motor mechanochemistry: cargo-driven unfolding (myosin V and kinesin-1) and cargo-driven dimerization (myosin VI). In the case of myosin V [41] and kinesin-1 [40], these intrinsically dimeric molecules exist in a folded, enzymatically and mechanically quiescent state in the absence of cargo, and in an extended, active state in the presence of cargo. In the case of myosin V, one such unfolding/activating cargo is melanophilin [77], while the unfolding/activation of kinesin-I requires an activator (FEZ1) in addition to a cargo (JIP1) [40]. Whether cargos simply trap the motor kinetically in its extended state or can allosterically induce the extended state remains an important unanswered question. Interestingly, fluorescence resonance energy transfer (FRET) studies showed that in folded kinesin-I, the motor’s light chains push the heads far apart, presumably to inhibit motility [40]. Other kinesin family members (e.g. kinesin-2, kinesin-7) are also subject to cargo-dependent unfolding/activation [40]. In the case of myosin VI, a quiescent, folded monomer can be converted to an extended processive dimer through interaction with dimeric cargo [9 ″″]. Dimerization may be facilitated by subsequent self association of the myosin VI heavy chains through weak coiled coil interactions, as well as by the sensing of membrane lipids by the CBD. Note that the properties of the medial tail of myosin VI (e.g. contribution to dimerization, lever arm length, reverse movement, etc) are currently areas of intense debate (see [43] for review). The cargo-unfolded, monomeric, non-processive version of myosin VI might also support certain cellular functions.